Method for increasing the elasticity of threads made of plastic materials



June 9, 1964 v vog 3,136,112

METHCD FOR INCREASfNG THE ELASTICITY OF THREADS MADE OF PLASTIC MATERIALS Filed Dec. 13, 1962 FIG. l

2 FIG. 20

. INVENZI OR VOJTECH VODICKA ATTORNEYS United States Patent f) 3,136,112 METHOD FOR INCREASING THE ELASTICITY F THREADS MADE OF PLASTIC MATERIALS Vojtech Vodika, Opladen, Germany, assignor to Dr. Plate G.m.b.H., Bonn, Germany, a firm Filed Dec. 13, 1962, Ser. No. 244,280 4 Claims. (Cl. 57-157) This invention relates to a method for increasing the elasticity of single threaded objects made of plastic materials, in particular of strings of tennis rackets consisting of materials such as polyamide, nylon, or the like.

The elasticity of strings of tennis rackets, which are made of polyamide, is usually increased by twisting the stretched thread under low tension and by fixing the twist thus achieved in that the string is subjected to heat, steam or hot water. Although this method increases the elasticity of the string its resistance to tearing is reduced if the twisting exceeds a certain degree. This means the string is of high tearing resistance but of insufficient elasticity where there is only little twisting. If twisting is increased to provide for good elasticity the string is soon overturned and therefore easily torn. For example, 150 turns per meter are too little to provide for sufficient elasticity. If the number of turns is increased to 300 per meter the string is elastic but it is overturned and due to its internal tension, easily tears when put into the tennis racket.

It is an object of the present invention to increase the elasticity of a string made of a plastic material without damaging its resistance to tearing.

Another object of the invention is to provide for a string for tennis rackets which is made of a plastic material but equals catgut strings in elasticity and resistance to tearing.

One manner by which the present invention may be practiced is in that the thread is at first twisted in one direction, whereupon it is flattened and released. The relaxed twisted thread is then twisted in opposite direction and stretched thereafter. Through the flattening, the thread twisted in one direction under low tension will partly keep its twist but its tension will be eliminated. As the released thread will have received screw-type turns its profile will be different to that of a thread which was not subject to treatment. Through the release, a new tensionless twist in opposite direction is created which has the shape of screw-type turns and stays without being fixed.

By the second twist, its direction being opposite to the first one, the screw-type turns of the thread will come to be closer to one another, and they will be fixed by the stretching following the second twist, so that, as a result, the thread will be of profiled, rough surface.

The thread treated in this manner has the twist necessary for increase of its elasticity without being subject to any internal tension worth mentioning which would diminish its resistance to tearing. Therefore, this thread is particularly suited for strings of tennis rackets.

In its first as well as in its second, adversely directed twisting treatment the thread may receive any number of turns and the number of turns may be equal or it may, without disadvantage, vary from one direction to the other. Also, the thickness of the thread as well as the degree of stretching may be freely chosen.

3,136,1 1? Patented June 9, 1964 A specific embodiment of the present invention is described hereinafter with reference to the accompanying drawing showing diagrammatically and by way of example an apparatus for carrying out the method of the present invention for increasing the elasticity of single thread tennis racket strings of nylon or. the like.

In the drawing:

FIGURE 1 is a side view of a device for carrying out the first twisting operation.

FIGURE 2 is a side view of a string flattening device.

FIGURE 2a illustrates diagrammatically the flattened string.

FIGURE 3 is a side view of a device for carrying out the second twisting operation.

A half-stretched round monofile string 1 of a diameter of 1.2 to 1.4 millimeters, consisting of a polyamide is held between two hooks 2 and 3 and under low tension twisted in left direction at turns per meter by turning the hook 3 in the direction of the arrow (FIG. 1). In this twisted state, the string 4 is guided between two cylindrical rollers 5 (FIG. 2) and flattened by them. Upon leaving the rollers, the string 6 opens itself out in opposite, i.e. right direction and looses its tension (FIG. 2a). The string will then have a profile the screw-type turns of which run to the right without the string being twisted as it is completely released. Thereupon, the released string 6 is held between two hooks 7 and 8 and is further twisted to the right, in the direction of the arrow (FIG. 3) which makes the windings of its screw-type profile be adjacent one another. To stick to the example, this right twist may again include 150 turns per meter. Ultimately, the string is stretched by pulling the hooks 7 and 8 in opposite directions so that its twist will be fixed. The finished string Will then have 300 turns into right direction per meter which make for its elasticity, but it will not be subjected to excessive internal tension. The string is not overturned and therefore exhibits strong resistance to tearing, while at the same time its intensity of twist assures increased elasticity.

The method according to the invention allows for both simple manufacture of the string and increased resistance to tearing together with good elasticity. The method may be further varied in that the polyamide thread may be twisted not only in a half-stretched state, but also in full stretch or without any stretch at all. The essential part of the method resides in the releasing of the thread subsequent to its first twisting as, by said releasing,

the string is made tensionless in conformance with its requirement for resistance against tearing, and yet, at the same time, due to the second twist, the direction of which is opposite to the first one, and which follows the flattening process the string has the twist necessary for the increase of its elasticity.

The above described methods are described to make the disclosure complete and to illustrate how the invention may be best carried out. It is not the intent that the invention be limited to the above described methods, as various changes may be made without departing from the spirit of the invention or the scope of the annexed claims.

I claim:

1. A method for increasing the elasticity of single threaded objects made of plastic materials, comprising first twisting the thread, thereupon flattening the twisted thread, releasing it, twisting it in direction opposite to the first twist, and stretching it.

2. A method for increasing the elasticity of strings of tennis rackets made of polyamide, comprising the steps twisting the string, thereupon flattening the thread in its twisted state, releasing of the flattened thread, thereafter twisting said thread in direction opposite to its first twist,

and finally stretching of the thread.

3. A method for improving the elasticity of strings of tennis rackets made of plastic materials, which comprises twisting the half-stretched thread, flattening of the twisted thread, releasing of the thread and subsequent twisting of said thread in direction opposite to the first twist, and finally stretching of the twisted thread.

2 4. A method as described in claim 3 in which the thread receives its first twist in an unstretched state.

References Cited in the file of this patent UNITED STATES PATENTS Burleson Sept. 13, 1960 Enneking July 3, 1962 Held July 3, 1962 Wegener et al. Apr. 16, 1963 FOREIGN PATENTS Great Britain, June 12, 1957 

1. A METHOD FOR INCREASING THE ELASTICITY OF SINGLE THREADED OBJECTS MADE OF PLATIC MATERIALS, COMPRISING FIRST TWISTING THE THREAD, THEREUPON FLATTENING THE TWISTED THREAD, RELEASING IT, TWISTING IT IN DIRECTION OPPOSITE TO THE FIRST TWIST, AND STRETCHING IT. 